In recent years, much attention has been attracted to magnetoresistive random access memories (MRAM) comprising magnetroresistive effect elements utilizing a magnetoresistive effect replacing a tunneling magneto resistive effect (TMR), as next-generation solid non-volatile memories.
One magnetroresistive effect element is a magnetic tunnel junction (MTJ) element with a three-layer stack configuration comprising a recording layer in which a magnetization direction is variable, an insulating film serving as a tunnel barrier, and a reference layer in which a predetermined magnetization direction is maintained.
The resistance of the MTJ element varies depending on the magnetization direction in the recording layer and the magnetization direction in the reference layer. The resistance of the MTJ element has a minimum when the two magnetization directions are parallel and has a maximum when the two magnetization directions are antiparallel These parallel state and antiparallel state are associated with binary information “0” and “1”, respectively, to allow the information to be stored.
Write of information to the MTJ element involves a magnetic-field write scheme in which only the magnetization direction in the recording layer is inverted by a current magnetic field resulting from a current flowing through a write wire and a write (spin injection write) scheme using spin angular momentum movement in which the magnetization direction in the recording layer is inverted by passing a spin polarization current through the MTJ element itself.
According to the former scheme, reduced element size increases the coercive force of a magnetic substance forming the recording layer. This tends to increase a write current, hindering the magnetic-field write scheme from achieving both miniaturization and reduced required current.
On the other hand, according to the latter scheme (spin injection write scheme), the amount of spin polarization electrons to be injected may decrease consistently with the volume of a magnetic layer forming the recording layer. Thus, the spin injection write scheme is expected to easily achieve miniaturization and reduced required current.
Furthermore, miniaturization of the MTJ element serves to reduce magnetic anisotropic energy required to maintain the magnetization direction in the recording layer constant. This disadvantageously prevents stored information from being maintained.
Techniques to increase the magnetic anisotropic energy include one utilizing shape magnetic anisotropy by, for example, increasing the aspect ratio of the MTJ element, the film thickness of the recording layer, or the saturation magnetization of the recording layer, and one utilizing a material with high magneto crystalline anisotropy.
The former technique is generally examined in connection with an in-plane magnetization type in which the axis of easy magnetization of a ferromagnetic substance corresponds to magnetization parallel to a film surface. However, this hinders miniaturization from being achieved, and further contributes to increasing an inversion current when the above-described spin injection write scheme is used.
On the other hand, the latter technique is advantageous to miniaturization and reduced required current. However, with the in-plane magnetization type, the axis of easy magnetization in the in-plane direction is significantly distributed within the film plane, leading to reduced TMR ratio.
To solve the above-described problems, every effort has been made to examine an MTJ element comprising a perpendicular magnetization magnetic film with an axis of easy magnetization perpendicular to the film plane (perpendicular magnetization MTJ element). Compared to the in-plane magnetization MTJ element, the perpendicular magnetization MTJ element enables a reduction in the distribution of the direction of the axis of easy magnetization, allowing a decrease in TMR ratio to be restrained.
As described above, the use of the perpendicular magnetization MTJ element based on the spin injection write scheme is expected to achieve both miniaturization and reduced required current. However, the perpendicular magnetization MTJ element with the perpendicular magnetization magnetic film involves a significant decrease in TMR ratio caused by a thermal load.